In a known embodiment of the prior art, described for example in the document EP 1 894 442, a mold with a self-contained heating system includes two mold bodies that define the forming cavity. Such a mold may be used for stamping operations or for hot molding operations. At least one of the mold bodies comprises a self-contained heating system using inductors. According to this exemplary embodiment of the prior art, the inductors are made up of electrical conductors extending in grooves or bores, forming closed cavities made in the mold body, which grooves or bores define the paths of the inductors. The paths of the inductors as well as their number and distribution in the mold body are determined by the shape of the cavity demarcating the molding recess made in said mold body, by the temperature to reach within that cavity and the distribution of temperature sought in said cavity during the stamping or molding cycle. Part of the mold body is made up of ferromagnetic material that is subjected to the effect of the inductors. This part of the mold body may be the entire mold, only a part of its volume, such as the part of the mold body in which the grooves or bores are made, or limited to the internal coating of the grooves or bores in which the inductors are located. Said mold is installed in a production environment, for example on the platens of a press. It is then connected to a high-frequency current generator available in the market. Said generator is connected to the inductors and heating is achieved by passing high-frequency alternating electric current in said inductors, which generates induced currents leading to the heating of the ferromagnetic part of the mold, which heats and transmits the heat by conduction to the cavity and finally to the material making up the future part made using the mold.
A high-frequency current generator operates by bringing into resonance the oscillating circuit made up of the inductor and the load heated by it. These conditions allow optimal inductive efficiency. When this condition is not fulfilled, the energy dispensed by the generator is consumed by Joule effect in the conductors that make up the inductors, said effect leading to no heating or too little heating of the ferromagnetic part. Thus, that lack of energy efficiency subjects the inductors to significant thermal stresses in view of the efficiency of the heating of the mold.
The document U.S. Pat. No. 1,948,704 describes an induction heating device suitable for the thermal treatment of material that is directly subjected to induction. For example, that device is suitable for the thermal treatment of a spring or for melting metal. The material heated by induction in this way is placed in an induction coil with known characteristics. The working conditions of the generator are adapted to the device, so that the coil and generator assembly makes up a resonant circuit. The introduction of the load in the circuit and the modification during the thermal treatment of the characteristics of the treated material by the fusion of the material or its being heated beyond the Curie point are liable to make the operating conditions of the device differ from the optimum conditions. Thus, the device disclosed in this document of the prior art comprises variable capacitances and inductances to adapt the response of the generator and try to always remain at the optimal conditions.
In the case of a tool with self-contained heating, the oscillating circuit, the shape of which is imposed by different technical constraints, is generally not resonant. Thus, when such a tool is connected to the high-frequency generator, in many cases, the generator can simply not start.
FIG. 1 of the prior art shows a schematic electric circuit of a self-contained induction heating device. The tooling circuit (120), corresponding to the inductors of the mold body interacting with said tooling is characterized by impedance Z1, combining the equivalent electrical resistances (105) and inductances (115) of the mold body and the inductors, R1 and L1. Also according to the prior art, a capacitor box (101) with adjustable capacitance C3 is connected to the generator (100) in parallel with the tooling circuit (120). The high-frequency electrical generator (100), characterized by impedance ZG, is placed in parallel in that circuit for powering it. The generator is adapted to supply alternating current within a set frequency range, generally located between 10 kHz and 100 kHz. The so-called load circuit made up by the tooling circuit and the capacitor box forms an oscillating circuit of the parallel type. To power said circuit, in optimal conditions, the power source comprises an electronic circuit that allows it to adjust itself automatically to the resonance frequency of the oscillating circuit.
As with any resonance phenomenon, it is characterized by a resonance frequency f0 and by a resonance peak width Δf, when it exists. The frequency f0 of the oscillating circuit resonance is given by the relationship:L1.C3.ω02=1, where ω0=2πf0
and the peak width is a function of the ratio L1/R1. The larger the value L1/R1, the narrower the resonance peak.
Thus, one condition for the generator starting up is that it is able to adjust to the resonance frequency of the oscillating circuit, that is to say that the resonance frequency is sufficiently marked by a narrow resonance peak and that the resonance frequency is located between the supply frequency range that said generator is capable of delivering. To that end, a quality factor Q=L1 ω0/R1 is defined. For the resonance peak to be marked and for the generator to be able to detect the resonance frequency of the oscillating circuit and adjust to it, it is necessary for that quality factor Q to be greater than or equal to 2. However, in most cases, said quality factor is substantially smaller than 1, so that the generator does not start and the adjustment of the capacitor C3 does not make it possible to modify the quality factor Q.
Besides, the power delivered by the generator and injected in the tooling circuit is maximal when the load impedance is in the output impedance range of the generator, or:Z1˜ZG 
But the values of R1 and L1 are chiefly determined by the geometry of the cavity and the technical constraints of temperature distribution in said cavity, so they only provide a small adjustment latitude.